The present invention relates generally to the field of powder metallurgy and in particular to compositions protecting nickel powder from agglomeration as well as a method for producing these compositions.
The introduction of extra-fine nickel powder to the powder metallurgy (P/M) industry has been delayed primarily due to agglomeration of the nickel powder. Specifically, commercial use of extra-fine nickel powder has been restricted due to the formation of agglomerates during mixing with other metal powders such as iron. The problem typically arises in the postproduction stage, such as during packaging and transport. These nickel agglomerates cause defects, which could lead to premature failure of a pressed and sintered steel part. Such steel parts are typically used in a variety of applications such as automotive components.
The potential of improving properties such as hardenability and density in powder metallurgy steels could be accomplished via improved nickel distribution and diffusion. Agglomerates create two principal problems. First, areas of high nickel content promote the formation of soft Ni-rich austenitic regions. These soft phases are detrimental to hardenability. Second, areas of weakness result from large agglomerates due to a lack of homogeneity leading to problems in parts such as differential shrinkage during sintering.
In order to make extra-fine nickel powder more suitable for the powder metallurgy industry, a method of preventing agglomeration is needed.
One solution employed by the hardmetal and diamond tooling industry is the use of high shear mixing technology such as plough shear blenders. Laboratory results using medium or high shear blending processes (V-cone blender with intensifier bar) have successfully reduced the number of agglomerates in powder metallurgy steels. However, most production blending facilities do not have this type of process equipment and low shear (double cone blenders) cannot break up the agglomerates. The ferrous-based “press and sinter” industry typically has low shear mixing technology, such as double-cone blenders and is unlikely to spend the capital needed for new mixers. Also, there is some concern that high shear mixing may alter the properties of the iron-based powder mixture; particularly with the longer mixing times needed to break up agglomerates.
Thus, there is a need in the nickel powder metallurgy industry for an inexpensive means to prevent agglomeration of nickel powder, particularly in steel blends and alloys where nickel is mixed with iron.
The issue of agglomeration is not a new one. The problems faced by agglomeration in powders in general is described in the background of U.S. Pat. No. 3,580,519, which is incorporated herein by reference. This patent teaches a process of grinding finely divided powdered substances with finely divided hydrophobic silica, or a process of mixing a ground powdered substance with a mixture of finely divided hydrophobic and hydrophilic silicas.
Fumed silica is known to enhance flowability of a metal oxide, as disclosed in U.S. Pat. Nos. 4,569,693 and 5,782,954.
Furthermore, U.S. Pat. No. 5,900,315 describes the preparation of a powder product with a fumed silica, and in particular a toner composition. The toner composition includes resin particles, at least one colorant, and a charge modified metal oxide which includes a metal oxide treated with a cyclic silazane. The composition is mixed by conventional mixing techniques.
However, a treatment to reduce or prevent agglomeration of extra-fine nickel powder, or an alloy blend comprising extra-fine nickel powder has not been disclosed or suggested in the prior art, thereby explaining the current need for a means for preventing agglomeration of nickel powders, particularly in alloy steel blends.